Known optical isolators comprise an input port and an output port at opposite ends of the device. Between the input and output port, a typical isolator has, in sequence, a polarization-dependent light beam division element, a polarization rotator, and a polarization-dependent light beam recombining element. These components are used for separating, rotating and recombining orthogonally polarized beams in such a way that light beams travel from the input port to the output port whereas light beams from the output port and travelling in the opposite direction are diverted so that they do not reach the input port.
Known optical circulators comprise additional input ports and/or output ports. A polarization-dependent division element, a first polarization rotator, a spatial "walk-off" or light beam displacement element, a second polarization rotator and a polarization mixing element are used to separate, rotate and combine orthogonally polarized beams to provide coupling between selected pairs of the input and output ports.
A disadvantage of such known non-reciprocal devices is the large number of components involved, and relatively long overall length.